1. Field of the Invention
The invention relates to a mains power supply unit for the purpose of supplying consumers that are connected directly or by way of a voltage system converter to a direct voltage branch of the mains power supply unit, comprising a buffer capacitor that can be charged from the mains supply and still supplies the direct voltage branch for a predetermined minimum period of time in the event of a failure or interruption in the mains supply.
2. Description of the Prior Art
In the case of electric energy converters, buffer capacitors are mostly provided, for example, in switched mode mains power supply units having an intermediate circuit voltage. In the case of a large mains supply voltage range, buffer capacitors of this type are to be dimensioned to the maximum voltage. Consideration must be given, however, to the fact that when there is a failure in the mains supply, the capacitors are only charged to the intermediate circuit voltage corresponding to the minimum mains supply voltage. Owing to the quadratic dependency of the energy content of a capacitor upon the voltage, the stored energy is well below the possible maximum, so that the buffer time becomes too short, for example, to make possible any further procedures of data protection for the consumer.
Regardless of the problem of the energy content, when supplying from alternating current mains supply, extreme distortions and corresponding high harmonic content in the alternating current mains supply may occur by virtue of the periodic, pulsed recharging of the capacitors.
Three different solutions are known for eradicating or reducing these problems:
a) Broaden the working range of the consumer (converter) to achieve lower supply voltages, whereby the buffer capacitors can be discharged to a lower voltage when the function of the consumer is maintained. The disadvantages of this solution are high development costs, as this almost reaches the boundaries of technical viability, inefficient use of the converter and that the distortions are not taken into consideration. PA1 b) With regard to the required energy content, dimension the buffer capacitor to the minimum supply voltage. This results in the buffer capacitor being considerably over-dimensioned and also results in only a part of the storage capacity being used. Weight, volume and price of the buffer capacitor take on undesired values and the distortions to the mains supply become even greater. PA1 c) By preconnecting a stabilised high-setting step, hold the voltage at the buffer capacitor at a fixed value irrespective of fluctuations in the mains supply voltage. This solution requires a dedicated switched mode mains power supply step, and solves the two problems stated above. However, it also demands a considerable outlay for the development and production, and to accept a larger space requirement.
EP 0 525 898 discloses a circuit that, by way of a capacitive voltage divider, obtains a feed current for the control circuit of a controlled switch, in order to enable the circuit to run-up during initiation of said circuit. Neither a buffer capacitor nor the controlled connection thereof is disclosed.
EP 0 622 889 A2 describes a switched mode mains power supply unit, which is to serve as a pre-controller, for example, for television apparatuses, and wherein the direct current at a load capacitor is controlled by means of an FET switching transistor so that this voltages is always substantially equal in the event of different alternating mains supply voltages, for example, 240 V and 120 V. The load capacitor is always connected in parallel with the load and cannot be considered to be a buffer capacitor.
The circuit disclosed in EP 0 223 316 A2 is a high-setting controller that has variable switching frequency and comprises a load capacitor that is always connected in parallel with the load, a capacitor of an LC input filter and a capacitor of an RC integration element, but not, however, a connectable buffer capacitor.
EP 0 256 569 A1 discloses a mains supply unit that comprises a longitudinal controller and is controlled so that the voltage ripple on the load moves towards zero, wherein the longitudinal controller transistor only has to obliterate the minimum power loss possible, since only the alternating voltage component is connected to said longitudinal controller transistor. In this case a load capacitor is also always connected in parallel with the load; no connectable buffer capacitors are revealed.